The technology of inspecting the inner part of a specimen using an X-ray transmission image in a non-destructive way has been essential in the field of medical and industrial non-destructive inspection. In particular, a flat panel detector (FPD) which directly captures an X-ray transmission image as electronic data has widely been used because of its rapid image taking, assistance for interpretation of radiogram by image processing, and so forth. As disclosed in Japanese Patent Application Laid-Open Publication No. H4-206573, an image sensor used in the FPD has a structure in which pixels at least including a photoelectric conversion element and a switching element are arranged in an array. In the X-ray image sensor which is currently most often used, a-Si photo diode (PD) is employed as a photoelectric conversion element and a-Si thin film transistor (TFT) is employed as a switching element.
In recent years, in the field of medical services, a means for carrying out highly precise X-ray fluoroscopy (means for obtaining an X-ray video image) has strongly been desired. This is because, in a catheter treatment, the position of the catheter needs to be accurately confirmed in real time. An image intensifier which is often used in X-ray fluoroscopy can only obtain insufficient spatial resolution in principle. Moreover, the current X-ray image sensor cannot easily operate at high speed. It is therefore difficult for the X-ray image sensor to carry out fluoroscopy at a high frame rate. The current X-ray image sensor is not capable of operating at high speed because the current-driving ability of the a-Si TFT which is a switching element is low. Meanwhile, as a means for making a switching element operate at high speed, Japanese Patent Application Laid-Open Publication No. 2006-165530 discloses a method of using an oxide semiconductor TFT as a switching element. The oxide semiconductor disclosed here includes, for example, an amorphous oxide containing In, Ga and Zn.
However, in the case where a TFT of an oxide semiconductor is used as a switching element in order to speed up the image sensor, such a problem occurs that the characteristic of the oxide semiconductor TFT is significantly varied at the time of manufacturing the image sensor. The inventors have analyzed the problem and found that the oxide semiconductor changes its property due to raw gas containing a large amount of hydrogen, which is used in forming a-Si PD, or due to the temperature in the process, and thereby varies the characteristic of the TFT.
To address this, the inventors have proposed in Japanese Laid-Open Publication No. 2015-90957 a structure in which a blocking layer for suppressing transmission of hydrogen is arranged between the oxide semiconductor TFT and the a-Si PD. A similar method is also disclosed in Japanese Patent Application Laid-Open Publication No. 2015-170859.
FIG. 25 is a section view illustrating the structure of an image sensor having an oxide semiconductor TFT in a related technology. An example disclosed in Japanese Patent Application Laid-Open Publication No. 2015-170859 will be described with reference to FIG. 25. In this example, a hydrogen barrier dielectric layer 37 is arranged on an oxide semiconductor TFT 11. A contact hole is formed in the hydrogen barrier dielectric layer 37. Through the contact hole, a source/drain layer 19 of the oxide semiconductor TFT 11 is electrically connected to a bottom diode contact 32 of a photosensitive element 31. Here, the bottom diode contact 32 is a metal layer containing Cr, Ti, W, Mo, Al, Nd-doped Al, Ta or a combination thereof. The hydrogen barrier dielectric layer 37 is a dielectric layer containing silicon nitride, silicon oxide, silicon oxide nitride, aluminum oxide, aluminum nitride, aluminum oxynitride, titanium oxide, tantalum oxide, titanium nitride, tantalum nitride, or a combination thereof. The bottom diode contact 32 and the hydrogen barrier dielectric layer 37 serve to prevent hydrogen at the time of forming the photosensitive element 31 from being diffused to the oxide semiconductor TFT 11.